Automated blood pressure monitoring has rapidly become an accepted and, in many cases, essential aspect of human healthcare. Such monitors are now a conventional part of the patient environment in emergency rooms, intensive and critical care units, and in the operating theater.
The oscillometric method of measuring blood pressure involves applying an inflatable cuff around an extremity of a patient's body, such as a patient's upper arm. The cuff is inflated to a pressure above the patient's systolic pressure and then the cuff pressure is reduced either continuously or incrementally in a series of small pressure steps. A pressure transducer measures the cuff pressure, including the pressure fluctuations resulting from the heart pumping activity that causes pressure and volume changes in the artery under the cuff. The data from the pressure transducer is used to compute the patient's systolic pressure, mean arterial pressure (MAP) and diastolic pressure.
An example of the oscillometric method of measuring blood pressure is shown and described in U.S. Pat. Nos. 4,360,029; 4,394,034; and 4,638,810, which are commonly assigned with the present invention.
During the use of a conventional NIBP monitoring system, the blood pressure cuff is placed around the arm of a patient and is inflated to an initial inflation pressure that fully occludes the brachial artery to prevent blood flow. The cuff is then progressively deflated from the initial inflation pressure and the pressure transducer detects pressure pulses associated with the patient's heartbeat as blood begins to flow past the pressure cuff. Typical blood pressure algorithms utilized within the NIBP monitor deflate the pressure cuff in a series of pressure steps determined by the algorithm used to operate the NIBP monitor. As an example, the pressure cuff is typically decreased in equal pressure steps of a fixed amount (e.g. 8 mmHg or similar value). The deflation of the blood pressure cuff occurs after an NIBP oscillation pulse amplitude has been recorded for the current pressure step. In this method, the cuff pressure deflation is not synchronized with the arrival of the next blood pressure pulse. Thus, the prior art algorithms do not optimize the timing of the cuff deflation.
During the deflation of the cuff pressure, the peak amplitude of the oscillation pulses detected by the system will normally increase from a lower level to a relative maximum, and thereafter decrease. These amplitudes form an oscillometric envelope for the patient. The cuff pressure at which the oscillation pulses have a maximum value has been found to be representative of the mean arterial pressure (MAP) of the patient. Systolic and diastolic pressures are then derived either as a predetermined fraction of the oscillation size at MAP, or by more sophisticated methods of direct processing of the oscillation complexes.
The step deflation technique as set forth in the Ramsey patents is the commercial standard of operation. A large percentage of clinically acceptable automated blood pressure monitors utilize the step deflation rationale. When in use, the blood pressure cuff is placed on a patient and the operator usually sets a time interval, typically from one to ninety minutes, at which blood pressure measurements are to be made. The non-invasive blood pressure (NIBP) monitor automatically starts a blood pressure determination at the end of the set time interval.
Generally, conventional NIBP monitors of the type described in the above-mentioned patents use oscillation pulse amplitude matching at each pressure step as one of the ways to discriminate good oscillations from artifacts and noise. In particular, pairs of oscillation pulses are compared at each pressure step to determine if the pulses are similar in amplitude and similar in other attributes, such as shape, area under the oscillation curve, slope, and the like. If the oscillation pulses compare within predetermined limits, the average pulse amplitude for the two pulses and the cuff pressure are stored and the pressure cuff is deflated to the next pressure level. However, if the oscillation pulses do not compare favorably, the attributes of the earlier oscillation pulse are typically ignored and the attributes of the later oscillation pulse are stored. The monitor does not deflate the blood pressure cuff and instead waits for another oscillation pulse to compare with the oscillation pulse stored. This process continues until two successive oscillation pulses are matched or a time limit is exceeded.
Although the step deflation technique described above can eliminate or reduce the effect artifacts have in the blood pressure determination, the step deflation technique typically requires the detection of two oscillation pulses during each pressure step. Even when detected oscillation pulses are very clean and artifact-free, the step deflation technique has an inherent delay in order to control the pressure level of each step. Therefore, the amount of time required to make the blood pressure determination will be extended by the time that the technique uses at each pressure step to control the pressure. It is desirable to provide a system and method to reduce the amount of time required to estimate a patient's blood pressure to enhance the performance of an NIBP system.